1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to high energy lasers.
2. Description of the Related Art
Four-wave mixing is a typical process for realizing a phase conjugation effect. See U.S. Pat. No. 5,726,795; issued Oct. 30, 1996 to A. A. Betin, M. S. Mangir, and D. A. Rockwell, entitled COMPACT PHASE-CONJUGATE MIRROR UTILIZING FOUR-WAVE MIXING IN A LOOP CONFIGURATION, the teachings of which are hereby incorporated herein by reference. Four-wave mixing involves the use of two counter propagating pump beams and a nonlinear medium. The beams propagate through the medium along with a signal beam of which a phase conjugate beam is to be generated. The signal beam propagates through the medium at an angle relative to at least one of the pump beams and thereby creates an interference pattern. The remaining pump beam reads the interference pattern as a hologram producing a phase conjugate return of the signal beam.
While any nonlinear medium may be used, for certain applications such as infrared lasers, e.g., CO2 lasers operating at 10.2 microns, fewer options are available. For these applications, thermal nonlinearities are currently typically used. Thermal nonlinearities are created by applying heat to conventional nonlinear media to create a temperature gradient that effects a change in the refractive index of the medium. The heat is generated by natural or controlled absorption of laser beam power while propagating through a nonlinear medium. The change in the refractive index leads to the creation of a hologram and thus a phase conjugate beam may be created.
Initially, long mediums were used to control undesirable temperature increases. However, this approach gave way to the use of a thin layer of liquid for nonlinear mediums for four-wave mixing.
Unfortunately, the efficacy of four-wave mixing for phase conjugation has been complicated by the need for high quality pump beams. The pump beam power is typically 100 times that of the signal beams. Accordingly, these beams must be properly conditioned to provide high quality diffraction limited (planar wave) beams. Thus, in order to compensate for distortions in the amplifier beamline, a phase conjugate of the signal beam must be generated. However, in order to generate a phase conjugate of the signal beam, the pump beams must be conditioned. Hence, this has limited the practical application of the four-wave mixing for optical phase conjugation.
Consequently, a loop type conjugator approach has been widely used because, although it uses a four-wave mixing process, it does not require high quality pump beams. As is well known in the art, in a typical loop phase conjugate mirror (LPCM) system, a beam is launched in a nonlinear medium, traverses a loop and re-enters the medium. With an amplifier in the loop, the loop will resonate due to spontaneous noise and generates a conjugated return beam. Hence, any type of nonlinearity may be used and it may be scaled for high power applications.
However, the cell, in which the nonlinear medium is disposed, in a loop PCM must be activated at high average power. Inasmuch as a thermal nonlinearity effect is being used, the cell will absorb thermal energy. Hence, heat must be rejected while the cell is being used to write and read holograms created therein. As a single thin layer nonlinear medium is insufficient to provide adequate heat rejection and angular selectivity, multiple layers are used to provide a volume hologram effect. Volume holograms are inherently selective of the conjugated beam relative to a noise component.
In one prior approach, a loop PCM is used with a thin layer nonlinear medium in which a liquid is induced to flow in the medium to optimize heat rejection. While viable, this approach was problematic for many current applications. For example, the hologram may be washed out if the liquid is moved too quickly. Further, the system is mechanical and, as such, costly to construct and operate. Hence, multiple layer holograms have not heretofore been found to be as effective as volume holograms in loop PCM applications.
Thus, a need remains in the art for an improved system or method for removing thermal energy from a liquid nonlinearity cell while retaining the phase conjugating properties thereof, including reflectivity and selectivity of a hologram created therein, which is scalable for high power applications.